New supplements claim to rev up your metabolism. Liz Krieger examines the latest evidence to determine if they really work.

We all know that woman who can eat whatever she wants without so much as a strain of the waistband or a bulge of a seam. "I just have a fast metabolism," she says with a smile. Well, since not everyone is so lucky — and certainly the metabolic slowdown around age 40 can affect even those previously immune — there will always be some new miracle pill claiming to amp up the rate at which your body naturally burns calories and fat. Here, experts weigh in on the ones with buzz, the ones that show promise, and the ones that show problems. Plus, easy ways to boost your metabolism every single day — sans supplements.

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GREEN-COFFEE-BEAN EXTRACT (GCB)

Sure, every dieter has heard that a jolt of caffeine can provide a temporary metabolism boost, but the big deal about this suddenly hot coffee supplement has nothing to do with the caffeine. "It's all about the chlorogenic acid, a natural component of coffee beans," says Lindsey Duncan, a nutritionist and naturopathic doctor with A-list clients like Marc Jacobs. "Harvested and processed raw — not roasted — GCB interferes with fat storage and glucose absorption." Specifically, it seems to get in the way of two enzymes that control fat-cell storage: lipoprotein lipase and hormone-sensitive lipase, says health and fitness guru Pam Peeke, a doctor and author of the new book The Hunger Fix. One recent study has kick-started a booming industry of GCB-hawking companies. In 22 weeks, 16 people who took GCB changed nothing about their diet yet lost an average of close to 18 pounds — more than 10 percent of their body weight — and 12 of 16 saw a heart-healthy reduction in their resting pulse. Perhaps equally remarkable, no side effects were reported.

But before you race out to gobble up this bitter bean, be aware that even a coauthor of the study has some quibbles with the research. Though he did not conduct the study himself, University of Scranton chemistry professor Joe Vinson analyzed the data provided by the principal researchers in India and now says that their methods were unorthodox. "They didn't compare people who took GCB to those who took a placebo," he explains. The daily dosage in the study was 700 or 1,050 milligrams, but Vinson also advises that what you may order from the Internet may not contain the same kind or quality of compound as the extract studied. Peeke is intrigued but offers similar concerns about what's actually in the capsules. "The study was small and short but still provocative," she says. "Like other phytochemicals in coffee, it can be beneficial if you're getting the best stuff." (Tip: Make sure the label states that the pills are at least 50 percent pure chlorogenic acid.) How much to take? Duncan — who, it must be noted, is also the CEO of Genesis Today, a major purveyor of GCB supplements — suggests taking at least two 400 milligram capsules with water 20 minutes before each meal.

Bottom line: Many experts say it can't hurt to try. "But I still wouldn't expect a magic bullet," says Keri Glassman, a New York nutritionist. There's also no word on whether the body habituates to GCB, causing it to lose effectiveness over time. And while GCB is the headline grabber now, this won't be the last you'll hear of substances that appear to work similarly, by "interfering with lipo-protein lipase and other fat pathways," says John J. Merendino Jr., chairman of endocrinology at Suburban Hospital in Bethesda, Maryland. "It's one of the things pharmaceutical companies are investigating as a way to battle obesity."

HUMAN GROWTH HORMONE (HGH)

One of the fastest ways to drop fat and add long, lean muscle is via a substance already coursing through your veins — human growth hormone. Ah, but there's always a catch! After the rapid growth of childhood and adolescence, your body doesn't make that much of it anymore. "When the soft body of a child morphs into the taut one of a teenager, that's thanks in part to HGH," Peeke says. It's the active ingredient in a number of FDA-approved drugs, mostly for kids who have a deficiency. But even though it is not approved for adults for weight-loss or antiaging purposes, shots and pills of HGH are being used off-label to speed up metabolism and shave off pesky midlife pounds at pricey "anti-aging clinics" and in some Hollywood circles. Trouble is, "this can come at the cost of a pretty major metabolic hit to your system," says Michael D. Jensen, an endocrinologist at the Mayo Clinic in Rochester, Minnesota. "We're talking diabetes, nerve damage from soft-tissue growth around nerves, heart problems, and, at the extreme end, pituitary tumors." If you buy HGH over the Internet, like some desperate dieters who aren't connected to doctors willing to dose it out off-label, you have no idea what you're getting or how it's manufactured, says Jensen. "This may very well be made in someone's bathtub. You simply don't want to mess around with this." Also, it's "frighteningly expensive," says Merendino. "You can spend thousands per month on the stuff."

Bottom line: Unless you have a true deficiency as well as an unlimited budget, taking HGH is "asking for trouble," says Peeke. Duncan suggests a few alternative ways to boost your own HGH naturally. Regular exercise and resistance training with heavy weights in particular can prompt your pituitary gland, which secretes HGH, to ramp up production in order to keep up with the demand of challenged muscles, he says. He also favors goji-berry juice as a "secretagogue" to "keep glands in the brain — pineal, pituitary — running smoothly."

RED RASPBERRY KETONES (RRK)

Red raspberries: a delicious sign of summer with a distinct ripe, sweet fragrance. For people in search of a slimmer silhouette, it's this berry's aromatic compound, synthesized in a pill, that's making headlines. The research is still rather nascent — and the testing has been entirely in vitro or on rodents, not humans — but it seems that "this compound increases secretion of a hormone associated with fat-cell metabolism called adiponectin," explains Peeke. In one study, lab mice given RRK in addition to a high-fat diet gained "significantly" less weight than those fed a high-fat diet alone and did not develop fatty liver disease — a compelling result, notes Duncan.

But obviously, mice are not men (or women), and for every rodent study that can be replicated in people, "I'll show you nine others that don't work out that way," says Jensen. And as with any compound that has a lot of "sensory effect" — meaning it smells or tastes very strong — "this can affect appetite. These mice can't tell you if they felt sick and had no appetite after eating the ketones," he says.

There's also little likelihood that what you are getting in pill form is truly the product of hundreds of potent little raspberries, harvested and processed to help you shed persistent pounds. "This is an easy compound to make in a lab," Merendino says. "What you're getting is most certainly synthetic."

Bottom line: As a weight-loss food, raspberries themselves are great — high in fiber and low in calories, says Merendino. But as a supplement? RRK is still a bit of a logical leap, and some human studies are needed. If you're going to give the capsules a go, Duncan advises taking at least two 400 milligram capsules with water 20 minutes before each meal.

BURN CALORIES—NATURALLY

There are a few proven, pill-free ways to boost your metabolism. It's no big surprise that exercise is key. "Just by getting up and moving around, not even exercising per se, you will burn double the calories you would sitting down," says Jensen. (You burn about one calorie per minute sitting on your duff, by the way. Strolling to the fridge at least burns two calories per minute.) When you do hit the gym, don't skimp on resistance training. The oft-repeated adage that muscle burns more calories than fat is indeed true, and the effect is not insignificant. Increasing lean body mass increases the number of calories you burn, says exercise physiologist Geralyn Coopersmith, national director of the Equinox Fitness Training Institute. "The best kinds of workouts are hard and fast but short," Coopersmith notes. "Research suggests that people who do high-intensity interval training burn more calories during and after workouts than people simply doing a steady jog or more basic strength moves." A half hour of tough training will do it, she adds. Also shown to keep your inner furnace humming along: getting enough sleep and eating a tip-top diet, says Duncan. Raspberries, anyone?